Centralize the implementation of move assignment operators

At the moment we have two main strategies for dealing with move
assignment in Qt:

1) move-and-swap, used by "containers" (in the broad sense): containers,
but also smart pointers and similar classes that can hold user-defined
types;

2) pure swap, used by containers that hold only memory (e.g. QString,
QByteArray, ...) as well as most implicitly shared datatypes.

Given the fact that a move assignment operator's code is just
boilerplate (whether it's move-and-swap or pure swap), provide two
_strictly internal_ macros to help write them, and apply the macros
across corelib and gui, porting away from the hand-rolled
implementations.

The rule of thumb when porting to the new macros is:

* Try to stick to the existing code behavior, unless broken

* if changing, then follow this checklist:

  * if the class does not have a move constructor => pure swap
    (but consider ADDING a move constructor, if possible!)

  * if the class does have a move constructor, try to follow the
     criteria above, namely:

    * if the class holds only memory, pure swap;

    * if the class may hold anything else but memory (file handles,
      etc.), then move and swap.

Noteworthy details:

* some operators planned to be removed in Qt 6 were not ported;

* as drive-by, some move constructors were simplified to be using
qExchange(); others were outright broken and got fixed;

* some contained some more interesting code and were not touched.

Change-Id: Idaab3489247dcbabb6df3fa1e5286b69e1d372e9
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

4 files changed, 8 insertions, 12 deletions

diff --git a/src/gui/image/qicon.h b/src/gui/image/qicon.h
index 964c29e785..fab8c3fe21 100644
--- a/src/gui/image/qicon.h
+++ b/src/gui/image/qicon.h
@@ -62,14 +62,13 @@ public:
     QIcon(const QPixmap &pixmap);
     QIcon(const QIcon &other);
     QIcon(QIcon &&other) noexcept
-        : d(other.d)
-    { other.d = nullptr; }
+        : d(qExchange(other.d, nullptr))
+    {}
     explicit QIcon(const QString &fileName); // file or resource name
     explicit QIcon(QIconEngine *engine);
     ~QIcon();
     QIcon &operator=(const QIcon &other);
-    inline QIcon &operator=(QIcon &&other) noexcept
-    { swap(other); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QIcon)
     inline void swap(QIcon &other) noexcept
     { qSwap(d, other.d); }
     bool operator==(const QIcon &) const = delete;
diff --git a/src/gui/image/qimage.h b/src/gui/image/qimage.h
index 89a696d5d6..d00c11b669 100644
--- a/src/gui/image/qimage.h
+++ b/src/gui/image/qimage.h
@@ -125,13 +125,12 @@ public:
 
     QImage(const QImage &);
     inline QImage(QImage &&other) noexcept
-        : QPaintDevice(), d(nullptr)
-    { qSwap(d, other.d); }
+        : QPaintDevice(), d(qExchange(other.d, nullptr))
+    {}
     ~QImage();
 
     QImage &operator=(const QImage &);
-    inline QImage &operator=(QImage &&other) noexcept
-    { qSwap(d, other.d); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QImage)
     inline void swap(QImage &other) noexcept
     { qSwap(d, other.d); }
 
diff --git a/src/gui/image/qpicture.h b/src/gui/image/qpicture.h
index 34860985ed..c3e622a1fe 100644
--- a/src/gui/image/qpicture.h
+++ b/src/gui/image/qpicture.h
@@ -77,8 +77,7 @@ public:
     void setBoundingRect(const QRect &r);
 
     QPicture& operator=(const QPicture &p);
-    inline QPicture &operator=(QPicture &&other) noexcept
-    { qSwap(d_ptr, other.d_ptr); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QPicture)
     inline void swap(QPicture &other) noexcept
     { d_ptr.swap(other.d_ptr); }
     void detach();
diff --git a/src/gui/image/qpixmap.h b/src/gui/image/qpixmap.h
index b30bef0f6b..e731597659 100644
--- a/src/gui/image/qpixmap.h
+++ b/src/gui/image/qpixmap.h
@@ -75,8 +75,7 @@ public:
     ~QPixmap();
 
     QPixmap &operator=(const QPixmap &);
-    inline QPixmap &operator=(QPixmap &&other) noexcept
-    { qSwap(data, other.data); return *this; }
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QPixmap)
     inline void swap(QPixmap &other) noexcept
     { qSwap(data, other.data); }
     bool operator==(const QPixmap &) const = delete;